This invention is related to printers, plotters and similar apparatus and, more particularly, in an apparatus which transports a web media by rotatable means and includes media position sensing means for developing a signal reflecting distance moved by the media, to a calibration device comprising deformable means for variably coupling the rotation of the rotatable means to the media position sensing means and means for adjusting the amount by which the deformable means is compressed so as to adjust the response of the media position sensing means to the rotation of the rotatable means.
Sensing and controlling paper travel using a shaft encoder is a well-known and difficult problem common among various devices, including printers, plotters, facsimile machines, and others. The problem is particularly acute in devices in which the quality of an image reproduced by the device on the paper depends upon the accuracy with which the paper travel speed or position is sensed. For this reason, calibrating the device so that it accurately senses the paper travel speed or position is critical to image quality.
Referring to FIG. 1, a typical printer according to the prior art controls paper travel with two pinch rollers 10, 12 which grip the paper 11 between them with sufficient force to prevent significant slippage of the paper. The pinch rollers 10, 12 rotate on shafts 14, 16, respectively, one of the shafts 14, 16 being rotated in turn by a stepper motor (not shown), for example. In order for the control system of the printer to accurately govern paper position and travel during printing, an optical encoder disk 18 is coaxially pressed onto the shaft 16 and rotates with the shaft. The optical encoder disk 18 has a series of small periodically spaced light admitting holes (not visible in the view of FIG. 1) lying in a circular locus near its peripheral circumference. An optical shaft encoder 20 has a light emitting diode 22 and a light sensing diode 24 facing each other across the periphery of the optical encoding disk 18. As the optical encoder disk 18 rotates, the holes in the disk 18 periodically permit light from the light emitting diode 22 to impinge on the light sensing diode 24. The light sensing diode 24 therefore produces a pulsed electrical signal representative of the motion of the pinch rollers 10, 12 and therefore of the paper position and travel therethrough. Of course, the control system of the printer must be designed to associate each pair of sequential pulses from the light sensing diode 24 with the corresponding exact amount of paper travel through the pinch rollers 10, 12.
The output of the light sensing diode 24 does not accurately reflect the position or rate of travel of the paper 11 through the pinch rollers 10, 12 if the diameter of either of the rollers 10, 12 or both of them is different from that assumed by the printer control system designer. This problem is unavoidable in most printers, particularly if the pinch rollers 10 or 12 have elastically deformable circumferential surfaces for achieving a firmer grip on the paper. The degree to which these elastic surfaces are compressed varies depending upon manufacturing tolerances, and therefore the effective diameter of the pinch rollers 10, 12 is unpredictable. The problem can be alleviated by providing a signal processor at the output of the light sensing diode and by calibrating the signal processor through a trial and error procedure until the printer control system interprets the output of the optical shaft encoder correctly. However, such a signal processor is expensive and the electronic calibration thereof is undesirably complex.
U.S. Pat. No. Re. 32,775 and U.S. Pat. No. 4,586,834 disclose the use of pinch rollers to actuate paper travel in a graphics plotter and in a printer, respectively. A pair of paper-gripping pinch rollers in which one of the rollers has a non-deformable surface and the other has an elastically deformable surface deformed by the non-deformable roller with which it is engaged is disclosed in U.S. Pat. No. 4,527,174 for providing a better grip on the paper to reduce paper slippage in a plotter. U.S. Pat. No. 3,093,284 discloses a similar pair of magnetic tape-gripping pinch rollers in which one roller is elastically deformable for maintaining different magnetic recording tape speeds past two pairs of capstans in a tape recorder.
U.S. Pat. No. 3,917,142, U.S. Pat. No. 3,963,110 and U.S. Pat. No. 3,857,471 disclose an optical shaft encoder, or the like, for sensing paper motion in a printer or typewriter. Typically, the optical shaft encoder is a disk driven directly by the pinch roller or the motor which is, in turn, moving the paper through the printer. The disk has periodically spaced holes through it along its circumference and is disposed between a light emitting diode and a light sensing diode (for example). A microprocessor computes the position of the paper as it travels through the pinch rollers from the output of the light sensing diode with an accuracy or resolution determined by the number of the periodically spaced holes along the disk circumference.
The accuracy with which the optical shaft encoder senses the paper position is reduced by diameter variations or eccentricities in the pinch rollers or other members of the linkage between the paper and the encoder disk. Because finite tolerances must be employed in manufacturing printers or plotters for a reasonable or competitive cost, there is always some diameter variation or eccentricity in the rollers and shafts, as well as other sources of error, which necessarily distort to some extent the paper position sensed by the optical shaft encoder. Of course, for a specific individual printer these sources of error are generally constant and therefore the signal produced by the optical shaft encoder can be corrected using a calibrated on-board signal processor or the like. Unfortunately, such a signal processor is relatively expensive and the time to calibrate it electronically would require a time consuming trial and error computation procedure performed by a skilled worker.
The problem to be solved, therefore, is to improve image quality in a printer or plotter by removing the sources of error which reduce the accuracy of the optical shaft encoder without requiring the use of expensive or time consuming calibration devices or procedures.
Accordingly, it is an object of the invention to provide an inexpensive means of accurately calibrating the optical shaft encoder of a printer or plotter.
It is a further object of the invention to provide an inexpensive means of changing the output of the optical shaft encoder so as to accurately compensate for variations from standard values in the diameter of rollers or pinch rollers governing the paper travel in a printer or plotter.
It is yet a further object of the invention to provide an inexpensive and purely mechanical means of changing the output of the optical shaft encoder so as to accurately compensate for variations from standard values in the diameter of rollers or pinch rollers governing the paper travel in a printer or plotter requiring only the turning of a single knob using a very simple calibration procedure.
Other objects and benefits of the invention will become apparent from the description which follows hereinafter when taken in conjunction with the accompanying drawings.